doc/README.fec_mxc: add documentation

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>

diff --git a/doc/README.fec_mxc b/doc/README.fec_mxc
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+U-boot config options used in fec_mxc.c
+
+CONFIG_FEC_MXC
+       Selects fec_mxc.c to be compiled into u-boot.
+
+CONFIG_MII
+       Must be defined if CONFIG_FEC_MXC is defined.
+
+CONFIG_FEC_XCV_TYPE
+       Defaults to MII100 for 100 Base-tx.
+       RGMII selects 1000 Base-tx reduced pin count interface.
+       RMII selects 100 Base-tx reduced pin count interface.
+
+CONFIG_FEC_MXC_SWAP_PACKET
+       Forced on iff MX28.
+       Swaps the bytes order of all words(4 byte units) in the packet.
+       This should not be specified by a board file. It is cpu specific.
+
+CONFIG_PHYLIB
+       fec_mxc supports PHYLIB and should be used for new boards.
+
+CONFIG_FEC_MXC_NO_ANEG
+       Relevant only if PHYLIB not used. Skips auto-negotiation restart.
+
+CONFIG_FEC_MXC_PHYADDR
+       Optional, selects the exact phy address that should be connected
+       and function fecmxc_initialize will try to initialize it.